The invention relates to a transparent panel-form loudspeaker utilizing a preselected number of transducers to excite a peripherally supported transparent panel to generate beneficial flexural vibrational mode shapes for radiating sound with desired pressure level over a specific frequency range. Conventional loudspeakers utilizing a cone-type membrane as a sound radiator have been widely used. The sound radiation of the conventional loudspeaker is achieved by attaching an electrodynamic type voice coil transducer to the smaller end of the cone-type membrane and using the transducer to drive the cone-type membrane to move back and forth. In general, an enclosure is necessary to prevent low-frequency waves from the rear of the loudspeaker, which are out of phase with those from the front, from diffracting around to the front and interfering destructively with the waves from the front. The existence of such enclosure makes the loudspeaker cumbersome, weighty, having dead corner for sound radiation and etc. The shortcomings of the conventional loudspeakers together with the impact of the rapid growth of flat display devices such as LCD and Plasma TV have led to the intensive development of panel-form loudspeakers in recent years and many proposals of making panel-form loudspeakers have thus been resulted. For instance, Watters used the concept of coincidence frequency, where the speed of flexural wave in panel matches the speed of sound in air, to design a light and stiff strip element of composite structure that can sustain flexural waves and produce a highly directional sound radiation over a specified frequency range. The opaqueness, highly directional sound radiation, and geometry of the long radiating panel have limited the applications of this type of panel-form loudspeakers. Heron designed a panel-from loudspeaker which had a resonant multi-mode radiation panel. The radiation panel was a skinned composite with a honeycomb core. At its corner there was a transducer used for exciting the plate to generate multi-modal flexural vibration with frequencies above the fundamental and coincidence frequencies of the panel and provide, hopefully, high sound radiation efficiency. The design of such radiating panel, however, makes it so stiff that it requires a very large and cumbersome moving-coil driver to drive the panel and its overall efficiency from the viewpoint of electrical input is even less than the conventional loudspeakers. Again the radiating panel of such loudspeaker is opaque and its applications are also limited. Recently, Azima et al have adopted the method of multi-modal flexural vibration in designing a panel-form loudspeaker with some specific ratios of length to width. In contrast to Heron's design, the transducer in this case is placed at a specific point near the center of the panel. The location of the transducer on the panel is chosen in such a way that the transducer is not situated at any of the nodal lines of the first 20 to 25 resonant modes and all the natural frequencies that have been excited in the selected frequency range are uniformly distributed. Although the panel-form loudspeakers designed using this method can produce sound with wider frequency range than those using the other previously proposed methods, there are still some shortcomings that may limit the applications of this panel-form loudspeakers. One of such shortcomings is that the near center location of the transducer can hinder viewers from seeing through the radiating panel even though the panel itself is transparent. Another major shortcoming of the panel-form loudspeaker is the existence of severe fluctuations in the spectrum of sound pressure level. For a panel under vibration, there may be several thousand resonant modes with frequencies falling in the range from 50 to 20 KHz. If the location of the transducer is merely determined using the first 20 to 25 resonant modes, it will be inevitable that some resonant modes in the middle and high frequency ranges will be over- or under-excited and this may lead to the formations of unfavourable peaks and pits in the sound pressure level spectrum of the panel-form loudspeaker. It also worths pointing out that another source contributing to the severe fluctuations in the sound pressure level spectrum is the interference of sound waves radiated from different regions on the panel radiator. For a vibrating panel, the sound waves radiated from the convex and concave regions on the panel surface are out-of-phase and can cause interference among them. If the sound interference of the panel vibrating at a specific frequency is serious, the sound pressure level at that frequency will be significantly lowered and thus cause a pit in the sound pressure level spectrum. The aforementioned difficulties, however, were not tackled by Azima et al. Therefore, in view of the shortcomings existing in the panel-form loudspeakers, it is apparent that the previously proposed methods for the design of the existing panel-form loudspeakers can only find limited applications and are unsuitable to be used in the design of transparent panel-form loudspeakers.
Recently, the rapid growth of flat display and mobile communication devices such as liquid crystal display (LCD) monitors, cellular phones and personal digital assistants (PDA) in usage have roused the urgent need for the research and development of transparent panel-form loudspeakers. Since the integration of transparent panel-form loudspeakers with flat display and mobile communication devices can greatly enhance the performance of such devices, it thus becomes important to have a method that can be used to design the desired transparent panel-form loudspeaker for the devices. In order to meet the need in the development of transparent panel-form loudspeaker, a method for the design of a transparent panel-form loudspeaker of high efficiency is presented in this invention. The detail descriptions of the method and the making of such transparent panel-form loudspeaker are given in the subsequent sections.